1. Field of the Invention
This invention relates to an improved method for the production of a synthesis gas that is suitable for the production of methanol. The improved method integrates a gasification process with steam methane reforming.
2. Description of the Prior Art
Methanol is one of the major chemical raw materials, ranking third in volume behind ammonia and ethylene. Worldwide demand for methanol as a chemical raw material continues to rise especially in view of its increasingly important role as a source of alternative energy, for example, as a motor fuel additive or in the conversion of methanol to gasoline.
The significant reactions in methanol synthesis are based on the equilibrium reaction of carbon oxides (CO and CO.sub.2) and hydrogen, in the direction of methanol formation as follows: EQU CO+2H.sub.2 +CH.sub.3 OH (1) EQU CO.sub.2 +3H.sub.2 +CH.sub.3 OH+H.sub.2 O (2)
Reactions (1) and (2) are exothermic and proceed with volume contraction. Therefore, maximum methanol yields generally occur at low temperatures and high pressures.
An important way for producing methanol is by first producing a synthesis gas from a methane-containing gas, such as natural gas. The synthesis gas can be generated using steam methane reforming, partial oxidation or gasification, or a combined reforming or autothermal reforming process.
Steam methane reforming is the catalytic reaction of natural gas with steam to produce a synthesis gas or "syngas", which includes H.sub.2, CO.sub.2, CO, CH.sub.4, and H.sub.2 O with an H.sub.2 to CO ratio of about 3:1 or higher. The steam methane reformation reaction is endothermic. Therefore, external heat is required. The natural gas and steam are typically fed into alloy tubes that contain a nickel based catalyst for the reforming reaction. The catalyst tubes are placed inside a refractory lined structure. A portion of the natural gas is used as fuel to provide the heat required for the reaction: EQU H.sub.2 O(g)+CH.sub.4 +3H.sub.2 +CO (3)
The drawbacks of steam methane reforming include its limitation to low pressure applications on the order of about 100-400 psig. Steam methane reforming also produces a syngas with a high CH.sub.4 impurity content in a range of about 3-15 percent, and requires the external supply of CO.sub.2 for methanol syngas requirements.
Partial oxidation or gasification is a non-catalytic reaction of natural gas with oxygen under controlled oxygen conditions. The reaction is exothermic as shown in the following reaction: EQU CH.sub.4 +1/2O.sub.2 .fwdarw.CO+2H.sub.2 (4)
The partial oxidation process can be operated at high pressure to minimize or eliminate the syngas compression needed to reach the desired elevated pressure suitable for methanol production, typically about 200-2000 psig. However, the syngas produced from the partial oxidation process has a lower H.sub.2 :CO ratio with little or no CH.sub.4 content. Typically, the CH.sub.4 varies from about 0-0.5 percent, and the H.sub.2 :CO ratio varies from about 1.5-2.0. As a result, external H.sub.2 would be needed to meet the methanol syngas requirements.
The combined reforming process uses a combination of conventional steam methane reforming, often referred to as "primary reforming", in combination with oxygenated catalytic reforming, often referred to as "secondary reforming", to generate stoichiometric ratioed synthesis gas for the production of methanol. See U.S. Pat. No. 4,888,130 to Banquy.
In a preferred aspect of the combined reforming process, a portion of the natural gas feedstock is fed to the primary reformer and the effluent is blended with the balance of the natural gas and oxygen prior to entering the secondary reformer. The drawback of the combined reforming process is that it is limited to moderate pressure applications, on the order of about 400 to 600 psig.
At higher pressures, reduced operating temperatures are necessary, and because increased amounts of CH.sub.4 are present in the feed to the secondary reformer, it is more likely that soot or carbon formation will be increased. This can damage or deactivate the catalyst and lead to greater feed consumption to produce the required amount of carbon monoxide.
Most commercial methanol synthesis plants operate in a pressure range of about 700-2000 psig using various copper based catalyst systems depending on the technology used. A number of different state-of-the-art technologies are known for synthesizing methanol, and are commonly referred to as the ICI (Imperial Chemical Industries) process, the Lurgi process, and the Mitsubishi process.
The methanol syngas, also referred to as "stoichiometric ratioed synthesis gas", from the syngas generation unit is fed to a methanol synthesis reactor at the desired pressure of about 700 to 2000 psig, depending upon the process employed. The syngas then reacts with a copper based catalyst to form methanol. The reaction is exothermic. Therefore, heat removal is ordinarily required. The raw or impure methanol is then condensed and purified to remove impurities such as higher alcohols including ethanol, propanol, and the like. The uncondensed vapor phase comprising unreacted methanol syngas is recycled to the feed.
The operation of compressing the methanol synthesis gas requires expensive equipment that is costly to maintain. Moreover, the need to compress the methanol synthesis gas to reach suitable operating pressures for the methanol synthesis operation further increases the production cost of methanol. Therefore, a process that produces stoichiometric ratioed synthesis gas at elevated pressures without the need for external compression would be very attractive to the industry.
For optimal methanol production, the stoichiometric ratioed syngas supplied to the methanol synthesis unit generally conforms to the following specifications: ##EQU1##